Reaction Pathway Diagrams (Cambridge (CIE) AS Chemistry): Revision Note
Exam code: 9701
Reaction Pathway Diagrams
A reaction pathway diagram shows the energies of the reactants, the transition state(s) and the products of the reaction with time
They are sometimes called energy profile diagrams
The transition state is a stage during the reaction at which chemical bonds are partially broken and formed
The transition state is very unstable – it cannot be isolated and is higher in energy than the reactants and products
The activation energy (Ea) is the energy needed to reach the transition state
We can define the activation energy as ‘the minimum amount of energy needed for reactant molecules to have a successful collision and start the reaction’
Example reaction pathway diagram
Exothermic reaction
In an exothermic reaction, the reactants are higher in energy than the products
The reactants are therefore closer in energy to the transition state
This means that exothermic reactions have a lower activation energy compared to endothermic reactions
The reaction pathway diagram for exothermic reactions
Endothermic reaction
In an endothermic reaction, the reactants are lower in energy than the products
The reactants are therefore further away in energy to the transition state
This means that endothermic reactions have a higher activation energy compared to exothermic reactions
The reaction pathway diagram for endothermic reactions
Worked Example
Drawing energy level diagrams of the combustion of methane
The Ea and ΔH for the complete combustion of methane are +2653 kJ mol-1 and –890 kJ mol-1 respectively.
Draw the reaction pathway diagram for this reaction
Answer
Step 1: The chemical equation for the complete combustion of methane is:
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l)
Step 2: Combustion reactions are always exothermic (ΔH is negative) so the reactants should be drawn higher in energy than the products
Step 3: Draw the curve in the energy level diagram, clearly showing the transition state
Step 4: Draw arrows to show the Ea and ΔH including their values
Worked Example
Determining the activation energy
Determine, using the reaction pathway diagram, the Ea for the forward and reverse reaction. The diagram shows the products 70 kJ mol-1 above the reactants, and the transition state 20 kJ mol-1 above the products.
Answer
The Ea is the energy difference from the energy level of the reactants to the top of the ‘hump’
Ea (forward reaction) = (+70 kJ mol-1) + (+ 20 kJ mol-1 ) = +90 kJ mol-1
As the question is asking for the reverse reaction, the Ea is the energy difference from the energy level of the products to the ‘hump’
Ea (reverse reaction) = +20 kJ mol-1
Examiner Tips and Tricks
The activation energy is the energy difference from reactants to the transition state.
The enthalpy change of the reaction is the energy difference from reactants to products.
Remember to label the axis of the reaction pathway diagrams!
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